The present invention relates generally to the field of gas analysis instrumentation, and more specifically to a combustion gas analyzer.
Industrial processes are used in the manufacture or combustion of various materials. It is often desirable to monitor operation of a process such that the process can be controlled and adjusted accordingly. Exhaust gas from the combustion is vented through a stack.
Combustion analyzers are used to measure the concentrations of a variety of exhaust gases in industrial combustion processes. For example, the exhaust gas in a combustion process consists of by-product and excess gases. The concentrations of exhaust gases, such as oxygen, oxides of nitrogen, sulfur dioxide and carbon monoxide, relate to the combustion efficiency of the process. Exhaust gas concentration measurements enable operators to adjust the amount of fuel supplied to the process to attain an efficient combustion.
Combustion oxygen analyzers are designed to measure the net concentration of excess oxygen in a combustion process. Excess oxygen is the oxygen remaining after all oxygen has been oxidized in the process and is related to the efficiency of the combustion process. An example of such a device is the Oxymitter 4000 manufactured and sold by Rosemount Analytical, Inc. of Orrville, Ohio. Common applications for a combustion oxygen analyzer include: glass furnaces, coking ovens, catalytic crackers, utility coal pulverizers, sulfur paint incinerators, and other industrial incinerators.
The combustion oxygen analyzer includes a sensor cell assembly which is positioned within an exhaust stack or duct which vents the exhaust gas from a combustion chamber. The sensor cell assembly includes a diffusion element and a sensing cell. As the exhaust gas is vented through the stack, it enters the sensor cell assembly and the diffusion element disperses the gas about the sensing cell. An electrical output from the sensor cell is indicative of oxygen concentration. Electrical circuitry in the transmitter reads the sensor cell output and provides an output related to oxygen concentration.
The combustion oxygen analyzer must be periodically calibrated in order to maintain accuracy in measurements. For example, the sensitivity of the sensor cell can drift over time. Calibration is through a process of standardizing the analyzer by determining the deviation between actual oxygen concentration and measured oxygen concentration. The deviation is used to adjust the output of the analyzer to bring it back into calibration. For example, to calibrate an oxygen analyzer, a calibration gas containing a mixture of oxygen and other gases has a known concentration of oxygen and is applied to the sensor cell assembly. The sensor cell assembly senses the concentration of oxygen in the calibration gas. The electrical circuitry provides an output value for the measured oxygen concentration. The measured value of oxygen is compared to the known concentration of oxygen in the calibration gas. A correction factor is calculated and can be applied to all subsequent measurements of the exhaust gas until a future calibration is performed. The correction factor can be stored, for example, in a memory in the transmitter.
In another calibration technique, the electrical circuitry in the transmitter measures impedance of the sensing cell to provide an indication of the accuracy of the sensing cell. An indication that the sensing cell is inaccurate can be used to indicate that calibration is required.
The calibration process typically requires the process to be shut down so that the analyzer can be removed from the stack for application of the calibration gas. Further, in applications where exhaust gas contains a high particle content, the diffusion element can become plugged and damaged. This also requires the industrial process to be shut down so that the diffusion element can be cleaned or replaced. Diffusion element maintenance and other procedures requiring the sensor cell assembly to be removed from service are time consuming and costly.